Field of Invention
This invention relates to the generation of electricity using photoemission and photoemission-thermionic hybrid generators.
Photovoltaic Solar Cells
The first practical solar cell was developed at Bell Laboratories in 1954 (U.S. Pat. No. 2,780,765). With the advent of the space program, photovoltaic cells made from semiconductor-grade silicon quickly became the power source of choice for use on satellites. Disruption of oil supplies to the industrialized world in the early 1970's led to serious consideration of photovoltaic cells as a terrestrial power source, focusing research attention on improving performance, lowering costs and increasing reliability. These three issues remain important today even though researchers have made extraordinary progress over the years.
For photovoltaic cells to be widely used, the costs must be competitive with those of conventional forms of electricity, which are typically 6-7 cents per kilowatt-hour. The US Department of Energy chose a target of 6 cents per kilowatt-hour for its terrestrial photovoltaic program (National Renewable Energy Laboratory. Photovoltaics Program Plan, FY 1991-FY 1995 (1991) National Photovoltaics Program, U.S. Dept. of Energy, Washington, D.C.). Today photovoltaics generate electricity at 20-30 cents per kilowatt hour; an improvement by a factor of five is therefore needed to compete in the bulk electricity market. A number of components influence photovoltaic energy costs. Foremost are the module efficiency, cost per unit area, and lifetime.
The maximum theoretical efficiency for a variety of semiconductor materials can be calculated and is in the 30-35% range, depending on the material. The highest-efficiency single-junction solar cells are made from crystalline silicon and GaAs. Silicon cells of 23% efficiency and GaAs cells of 25% efficiency have been reported. The efficiency of polycrystalline silicon is approximately 18%. A feature of photovoltaic cells is the need to draw current by way of metal contacts distributed over the negative and positive faces of the cell. This creates a problem for cell efficiencies because the contacts create an area that shades the semiconductor material. One approach to avoiding this problem is to use electrodes formed on transparent surfaces. For example, in U.S. Pat. No. 4,694,116 to Yutaki et al., entitled "Thin-Film Solar Cell", a thin-film solar cell is described which has a two-layered transparent electrode formed on a transparent substrate, a photoelectric conversion section formed on the transparent electrode, and a back electrode formed on the photoelectric conversion section. The photoelectric conversion section is generally constituted as a P/N junction. This device has efficiencies in the 7-9% range, depending on the manner of fabrication. Cells made using the edge-defined film-fed growth-ribbon process are reported to have efficiencies of 14% and the figure for dendritic web cells is 15.5%. The highest thin-film cell efficiency reported is 15.8%, for cadmium telluride. Thin films of silicon on ceramic substrates have yielded efficiencies of 15.7%.
In terms of cost, the cheapest material is silicon. There are three major types of silicon solar cells. The first type uses amorphous silicon. These cells do not possess a regular crystal structure. They can be produced in films of 0.3 microns and their production is relatively simple and cheap. Mass production of these cells is quite easy, with small amounts of material being deposited on to a substrate such as glass or aluminum, and can even be made flexible. Their major drawback is their low efficiency and short life-span. Presently, efficiencies are only 5-7%, however this drops to 3-4% in operation due to amorphous silicon instabilities. It is generally used for small scale applications such as calculators and watches.
The second type uses poly-crystalline silicon. These cells consist of a number of silicon crystals grown as an ingot from which wafers are cut. Their maximum efficiency is generally 15% and is standard material for high output applications. These wafers are of the order of 250-400 microns thick.
The final type uses mono-crystalline silicon. These are the highest efficiency silicon cells available and are cut from carefully grown ingots consisting of one crystal only. However in the past their expense has generally precluded them from anything except "space" applications where mass and area limitations are important. Recent developments in mono-crystalline cell technology has seen their cost reduced to around that of the poly-crystalline cell.
An important criterion is therefore cost per watt capacity, which is a compound of the cost per unit area and efficiency. For example crystalline silicon devices cost about 3.5$/W peak and have an efficiency of about 13%. This means that such devices will cost around 450$/m.sup.2. On the other hand, amorphous silicon devices, with efficiencies around 5% cost about 2.5$/W peak, or about 125$/m.sup.2. So a 10 m.sup.2 array will generate 1.3 kW peak for the crystalline silicon device and 0.5 kW peak for the amorphous silicon device, and they will cost $4500 and $1250 respectively. The low efficiency device is thus more economical.
Cells other than silicon are available and use materials with band-gaps nearer to 1.5 eV, such as GaAs and CdTe. They have higher theoretical efficiencies because their particular band-gap energies are closer to the theoretical optimum than silicon, which has a band-gap of 1.12 eV. They do however use materials which are more expensive, less abundant, and can be environmentally hazardous.
Efficiencies are increased as the light intensity is increased, and photovoltaic cells may be used in concentrator systems where the sun's radiation is focused onto the device using a reflector. This advantage is offset by the need to provide a cooling system, because performance and stability degrade at high temperatures.
There remains a need therefore for devices which are inexpensive to produce, and which exhibit stable operation at elevated temperatures in concentrator applications.
Photocells
These devices comprise a photocathode and an anode. The anode is small, often no more than a wire. It is maintained at a positive potential to attract electrons emitted from the cathode. When light impinges on the photocathode, electrons are released which move to the anode. This flow of electrons effectively reduces the resistance of the device, allowing a current to flow in the external biasing circuit. The magnitude of the current is dependent on the intensity of the incident light. These devices do not generate electricity.
Thermionic and Photoelectric Cells
In U.S. Pat. No. 4,266,179 entitled Solar Energy Concentration System", Hamm teaches that if solar energy is used as the heat source for a thermionic converter, it needs to be concentrated so that the temperature at the thermionic converter exceeds 2,800.degree. K. He goes on to describe multiple reflector units that are able to vary the energy concentration at the transducer by 20,000 to 250,000 fold. When this system is used with the "Radiant Energy to Electrical Power Conversion System" described by Brunson in U.S. Pat. No. 4,188,571 an output of 1.38 kW is projected from a device having electrodes separated by 6.3 .mu.m and a cathode temperature of 3630 Kelvin. Both of these inventions teach that thermionic converters may be used to harness solar energy only if a concentrator is used: they do not teach that thermionic solar energy conversion may be achieved at ambient temperatures. Furthermore, such devices operate by the conversion of light energy to heat, thence the conversion of heat to electricity. They do not teach the direct use of photon energy.
U.S. Pat. No. 4,168,716 to Fowler and Israel, entitled "Solar-Powered Thermionic-Photoelectric Laser", describes a solar-powered thermionic-photoelectric current generator which employs a parabolic telescope for collecting and concentrating sunlight into a narrow beam which is incident upon a thorium-doped tungsten cathode target within an evacuated envelope. This invention uses both thermionic and photoelectric emission from a target to enhance the current generating capabilities of a physical system which might utilize either effect alone. The device is designed so that the space charge of electrons is continuously swept away from the target by the radiation pressure of the light incident upon the target at very large angles of incidence. This invention does not use close spaced electrodes; rather it relies on the radiation pressure of the light to overcome space charge effects. Again this invention teaches that a solar energy concentrator is required in order to use a thermionic-photoelectric generator for harnessing solar energy. This invention also teaches that photoelectric emission is an ancillary source of electrons. The invention does not teach that photoelectric emission is sufficient of itself for the efficient generation of electricity.
Another invention using light to improve the efficiency of thermionic converters is U.S. Pat. No. 3,300,660 to Bensimon, entitled "Thermionic Energy Converter with Photon Ionization". Bensimon describes a device having a capillary emitter, the channels of which permit the penetration of light energy from an external source in order to facilitate the ionization of the atoms of an ionizable material, such as cesium, employed to overcome space charge effects. In this invention, photoelectric emission is not a contributing factor.
It is clear from the above that the art does not teach that pure photoelectric emission can be used for power generation. Where photocathodes have been described, they are for instrument use. For example in U.S. Pat. No. 5,598,062 to Iigami, entitled "Transparent Photocathode" a transparent photocathode is described which composed of a silver layer formed on a transparent substrate, comprising silver particles having an average diameter of 80 to 200 nm, and a silver oxide layer, potassium layer, and a cesium layer. As a result of the silver layer comprising silver particles having dispersive diameters, the transparent photocathode can selectively achieve high sensitivity to an infra-red region of near 1500 nm, and may be thus used in an infra-red analyzer. Although the photocathode is transparent, this invention does not teach that this is an advantage for use in harnessing solar energy for the use of electricity. Indeed, these devices are used for light detection, and as such consume power.
Another invention using a transparent electrode, this time a transparent collector or anode, is described in U.S. Pat. No. 5,028,835 to Fitzpatrick, entitled "Thermionic Energy Production". This invention describes a thermionic device having transparent collector surfaces coated with a thin film of conductive material. This arrangement reduces the conduction of heat by radiation from the hot emitter, thereby increasing the efficiency of the device. It is not taught that the transparent collector directly aids in the generation of electricity from solar energy.
Thus it can be seen from the foregoing that the use of the photoelectric effect to harness solar energy for electricity generation is known only as an adjunct to thermionic emission.
Generation of electricity from solar energy using a device relying on the photoelectric effect alone is not known to the art.
It can also be seen that previous devices using thermionic emission for electricity generation have required the use of solar concentrators to generate high temperatures.